The bacteriophage T4 is a powerful tool in the analysis of basic mechanisms of mutagenesis. The genetic information of rII region has been utilized extensively, resulting in the proposition of specific mutagenic pathways. Direct confirmation of predictions made by relying on T4 genetic data is now possible using DNA cloning and sequencing. Since analysis of large numbers of mutants is involved, a technique for sequencing specific regions of T4 DNA without prior cloning of each individual mutant fragment is being developed.